The invention concerns a method and apparatus for introducing an optical signal from a first light conductor or first optical fiber, into a second light conductor or second optical fiber, whereby the optical signal is radiated through a buffer into the cladding and the core of the first optical fiber and the optical signal from the second optical fiber is detected through the buffer, and wherein the ends of the optical fibers are arranged relative to each other in such manner that the greatest possible part of the optical signal is passed from the end of the first optical fiber into the end of the second optical fiber. Such an arrangement is necessary before the ends of the optical fibers are interconnected by welding or bonding.
An optical fiber can generally be described as including a core of high-grade translucent material such as doped quartz, around which is formed a translucent cladding material, such as quartz, preferably having a refractive index lower than that of the core. The cladding is in turn surrounded by a buffer usually consisting of a polymeric material. The refractive index of the buffer can be higher than, lower than, or equal to that of the cladding.
Optical fibers currently used for data transmission can be divided into two main groups, these groups being (1) multimode optical fibers, and (2) monomode optical fibers.
A multimode optical fiber generally has a core having a diameter in the order of magnitude of 50 .mu.m and a cladding diameter in the order of 125 .mu.m. In order to keep the ratio between the cladding diameter and the core diameter low, the core is arranged substantially concentrically within the jacket so that no great problem arises in aligning two optical fiber ends with one another. The monomode optical fibers presently in use comprise a core having a diameter in the order of magnitude of 8 .mu.m and a cladding diameter of approximately 125 .mu.m. Since the core diameter is small in relation to the cladding diameter, the core often does not lie quite concentrically within the cladding. This results in alignment problems, since the amount of light in the core of a monomode filament is less than that in a multimode fiber when the same light source is used.
In order to reduce as much as possible the attenuation that occurs at a weld or bond between the two ends of monomode fibers, it is necessary to align the cores of the fibers very accurately with each other. The alignment is preferably achieved by passing an optical signal into the core of the first fiber and by passing the light emerging from the end of the first fiber into the end of the second fiber so that maximum transfer of the optical signal is achieved. In this connection it is necessary that the optical signal be located principally in the core and that the amount of light in the cladding be minimized, since otherwise precise alignment of the cores does not occur.
European Patent No. EP30-106 discloses a method of aligning two monomode fibers with each other. An optical signal is introduced at a first end of the first fiber, whereafter a second end of the first fiber is so positioned in relation to a first end of the second fiber that maximum transfer of signal occurs. Detection of the signal in the second fiber is carried out by detecting the light in the cladding.
The method in accordance with this proposal is disadvantageous since the radiation or propagation of the signal in the first fiber occurs through the length of the fiber over a great distance. The proposed out-coupling method is successful only in the case of fibers having a buffer with a refractive index that is lower than that of the cladding.
Netherlands Patent Application No. NL-A-8201741 discloses a method and apparatus which render it possible to radiate an optical signal at a short distance from the coupling point. In this apparatus, however, no steps are taken to ensure that the signal in the core is as great as possible and that the signal in the cladding is as small as possible. Although the proposed method is quite suitable for alignment of fiber claddings which is sufficient when multimode fibers are being spliced, this method is not suitable when monomode fibers are being spliced since alignment of the claddings does not ensure that the monomode cores will be aligned.